There has been known a semiconductor device having a GaN device formed using a compound semiconductor mainly composed of GaN. When the GaN device is to be formed, a natural oxide film is formed on the surface of a GaN layer due to exposure to the air, and hence it is necessary to remove the natural oxide film before formation of a gate insulating film. However, when the natural oxide film is removed by hydrofluoric acid, damage is left on the GaN layer.
Accordingly, Patent Literature 1 discloses a GaN device forming method capable of reducing this damaged layer. Specifically, there is performed a step of forming on a first semiconductor layer a first sacrificial layer in which solid solubility of impurities is higher than in the first semiconductor layer made up of the GaN layer. There is then performed a step of annealing the first sacrificial layer and the first semiconductor layer. Subsequently, a step of removing the first sacrificial layer by wet etching is performed, and thereafter, at least either a step of covering at least part of the first semiconductor layer with an insulator layer or a step of etching part of the semiconductor layer is performed, to remove the damage on the surface of the first semiconductor layer. Thereafter, an electrode layer electrically connected to the first semiconductor layer is formed. In such a manner, the damage formed on the surface of the GaN layer is removed.
Further, there is also a method where at the time of producing a structure with a top layer laminated on the surface of the GaN layer, the natural oxide film formed on the surface of the GaN layer is removed so as not to damage the surface. Specifically, there is performed a step of exposing the surface of the GaN layer to a gas containing ammonia in a non-plasma state. Thereafter, there is performed a step of laminating a silicon oxide (SiO2) layer without exposing the surface of the GaN layer, exposed to the gas containing ammonia, to the air. In such a manner, after the natural oxide film has been removed using the gas containing ammonia, the silicon oxide film is removed on the surface of the GaN layer before formation of the natural oxide film, to thereby reduce the damage on the GaN layer.
However, even in either of the above cases, a donor element concentration (Si concentration or O concentration) in a surface layer section of the GaN layer cannot be reduced. Hence, Si or O present in the surface layer section of the GaN layer acts as a donor element to cause a variation in threshold voltage of the GaN device.